1. Field of the Invention
The present invention relates to a dielectric ceramic composition used as a dielectric layer of a for example multilayer ceramic capacitor etc., an electronic device using this dielectric ceramic composition as a dielectric layer, and a method for producing the same.
2. Description of the Related Art
A multilayer ceramic capacitor, one example of an electronic device, is formed by printing a conductive paste on to a green sheet comprised of a predetermined dielectric ceramic composition, stacking a plurality of such green sheets printed with the ceramic paste, and firing the green sheets and internal electrodes together.
Conventional dielectric ceramic compositions had the property of being reduced and being made semiconductive by firing in a low oxygen partial pressure neutral or reducing atmosphere. Therefore, when producing a multilayer ceramic capacitor, one was forced to fire in a high oxygen partial pressure oxidizing atmosphere. Along with this, it was necessary to use, as the material of the internal electrodes fired simultaneously with the dielectric ceramic composition, an expensive precious metal which would not melt at the temperature of sintering of the dielectric ceramic composition and would not be oxidized even if fired under an oxidizing atmosphere (for example, palladium, platinum, etc.) This proved to be a major obstacle in reducing the price of the multilayer ceramic capacitors produced.
In the face of this, for use of an inexpensive base metal (for example, nickel, copper, etc.) as the material of the internal electrodes, it is necessary to develop a dielectric ceramic composition which does not become semiconductive even if fired under a neutral or reducing atmosphere at a low temperature, that is, is superior in resistance to reduction, and has a sufficient dielectric constant and superior dielectric characteristics (for example, small rate of change of temperature coefficient of capacitance) after firing.
In the past, various proposals have been made for dielectric ceramic compositions enabling use of a base metal for the material of the internal electrodes.
For example, Japanese Unexamined Patent Publication (Kokai) No. 1988-224108 discloses a dielectric ceramic composition containing as a main component a dielectric oxide of a composition expressed by (Sr1xe2x88x92xCax)m(Ti1xe2x88x92yZry)O3 (where, 0.30xe2x89xa6xxe2x89xa60.50, 0.03xe2x89xa6yxe2x89xa60.20, 0.95xe2x89xa6mxe2x89xa61.08) and containing as subcomponents, with respect to 100 parts by weight of this main component, 0.01 to 2.00 parts by weight of Mn converted to MnO2 and 0.10 to 4.00 parts by weight of SiO2.
Further, Japanese Unexamined Patent Publication (Kokai) No. 1988-224109 discloses a dielectric ceramic composition containing, with respect to the above main component, 0.01 to 1.00 part by weight of Zn in addition to the Mn and SiO2.
Further, Japanese Unexamined Patent Publication (Kokai) No. 1992-206109 discloses a dielectric ceramic composition containing as a main component a dielectric oxide of a composition expressed by (Sr1xe2x88x92xCax)m(Ti1xe2x88x92yZry)O3 (where, 0.30xe2x89xa6xxe2x89xa60.50, 0.00xe2x89xa6yxe2x89xa60.20, 0.95xe2x89xa6mxe2x89xa61.08) and having a particle size of the powder in the range of 0.1 to 1.0 xcexcm.
Further, Japanese Examined Patent Publication (Kokoku) No. 1987-24388 discloses a dielectric ceramic composition containing as a main component a dielectric oxide of a composition expressed by (MeO)kTiO2 (where, Me is a metal selected from Sr, Ca, and Sr+Ca and k is 1.00 to 1.04) and containing as a glass component, with respect to 100 parts by weight of this main component, 0.2 to 10.0 parts by weight of Li2O,M (where, M is at least one type of metal oxide selected from BaO, CaO, and SrO) and SiO2 used in a predetermined molar ratio.
Further, Japanese Patent No. 2508359 (Japanese Unexamined Patent Publication (Kokai) No. 1992-14704) discloses a dielectric ceramic composition containing as a main component a dielectric oxide of a composition expressed by (Sr1xe2x88x92xCax)m(Ti1xe2x88x92yZry)O3 (where, 0.35xe2x89xa6xxe2x89xa60.41, 0 less than yxe2x89xa60.1, m=1.00) and containing as subcomponents, with respect to 100 parts by weight of this main component, 0 part by weight to 3.0 parts by weight, exclusive, of SiO2.
Further, Japanese Examined Patent Publication (Kokoku) No. 1993-18201 discloses a dielectric ceramic composition containing as a main component a dielectric oxide of a composition expressed by (Sr1xe2x88x92xCax)m(Ti1xe2x88x92yZry)O3 (where, 0 less than x less than 1.0, 0.005xe2x89xa6yxe2x89xa60.10, 1.00xe2x89xa6mxe2x89xa61.04) and containing as subcomponents, with respect to 100 parts by weight of this main component, specific ranges of Li2O, SiO2, and MO (where, MO is at least one metal oxide selected from BaO, MgO, ZnO, SrO, and Ca).
Further, Japanese Examined Patent Publication (Kokoku) No. 1996-24006 (Japanese Unexamined Patent Publication (Kokai) No. 1988-224106) discloses a dielectric ceramic composition containing as a main component a dielectric oxide of a composition expressed by (Sr1xe2x88x92xCax)m(Ti1xe2x88x92yZry)O3 (where, 0.30xe2x89xa6xxe2x89xa60.50, 0.03xe2x89xa6yxe2x89xa60.20, 0.95xe2x89xa6mxe2x89xa61.08) and containing as subcomponents, with respect to 100 parts by weight of this main component, 0.01 to 2.00 parts by weight of Mn converted to MnO2, 0.10 to 4.00 parts by weight of SiO2, and 0.01 to 1.00 part by weight of MgO.
The dielectric ceramic compositions of these publications, however, all suffered from the problems of a short accelerated life of the insulation resistance after firing and of a lower reliability of the multilayer ceramic capacitor obtained when producing a multilayer ceramic capacitor using the dielectric ceramic compositions and having internal electrodes of nickel or another base metal.
A first object of the present invention is to provide a dielectric ceramic composition which is superior in resistance to reduction at the time of firing, has a superior capacity-temperature characteristic after firing, and enables an improvement in the accelerated life of the insulation resistance.
A second object of the present invention is to provide a chip capacitor or other electronic device having a superior capacity-temperature characteristic improved in accelerated life of the insulation resistance, and enhanced in reliability.
A third object of the present invention is to provide a method of producing an electronic device enabling improvement of the defect rate of the initial insulation resistance while maintaining the reliability sought for the electronic device.
To achieve the above first object, there is provided a dielectric ceramic composition comprising at least a main component containing a dielectric oxide of a composition expressed by {(Sr1xe2x88x92xCax)O}mxc2x7(Ti1xe2x88x92yZ ry)O2 and a first subcomponent containing at least one type of compound selected from oxides of V, Nb, W, Ta, and Mo and/or compounds forming these oxides after firing, wherein the symbols m, x, and y showing the molar ratio of the composition in the formula contained in the main component are in relations of
0.94 less than m less than 1.08,
0xe2x89xa6xxe2x89xa61.00, and
0xe2x89xa6yxe2x89xa60.20 and
the ratio of the first subcomponent with respect to 100 moles of the main component, which is converted to the metal element in the oxide, is 0.01 molexe2x89xa6first subcomponent less than 2 moles.
Preferably, the dielectric ceramic composition according to the present invention further comprises a second subcomponent containing an oxide of Mn and/or a compound forming an oxide of Mn after firing, wherein the ratio of the second subcomponent with respect to 100 moles of the main component, which is converted to the metal element in the oxide, is 0 molexe2x89xa6second subcomponent less than 4 moles.
Preferably, the dielectric ceramic composition according to the present invention further comprises a third subcomponent containing at least one type of compound selected from SiO2, MO (where, M is at least one element selected from Ba, Ca, Sr, and Mg), Li2O, and B2O3, wherein the ratio of the third subcomponent with respect to 100 moles of the main component, which is converted to oxide, is 0 mole less than third subcomponent less than 15 moles.
Particularly preferred embodiments of the third subcomponent are as follows:
More preferably, the dielectric ceramic composition according to the present invention further comprises a third subcomponent (Srp, Ca1xe2x88x92p)SiO3 (where, p is 0.3xe2x89xa6pxe2x89xa61), wherein the ratio of the third subcomponent with respect to 100 moles of the main component, which is converted to oxide, is 0 mole less than third subcomponent less than 15 moles. This type of third subcomponent is considered to function as a sintering aid.
Preferably, the dielectric ceramic composition according to the present invention further has a rate of change of the electrostatic capacity with respect to temperature (xcex94C) of xe2x88x922000 to 0 ppm/xc2x0 C. at least in the temperature range of 20 to 85xc2x0 C., preferably xe2x88x921500 to 0 ppm/xc2x0 C., more preferably xe2x88x921000 to 0 ppm/xc2x0 C. Here, the reference temperature of the electrostatic capacity C is 20xc2x0 C.
To achieve the above second object, according to the present invention, there is provided an electronic device having a dielectric layer, wherein the dielectric layer is comprised of a dielectric ceramic composition and the dielectric ceramic composition is comprised of any of the above dielectric ceramic compositions.
Preferably, the electronic device according to the present invention is provided with a capacitor device body comprised of the dielectric layers and internal electrode layers alternately stacked.
Preferably, the electronic device according to the present invention has a conductive material contained in the internal electrode layers comprised of nickel or a nickel alloy.
To achieve the third object, according to the present invention, there is provided a method of producing an electronic device comprising the steps of preparing a dielectric paste using any one of the above dielectric ceramic compositions; preparing a paste for internal electrodes; alternately stacking the dielectric paste and the internal electrode paste to obtain a multilayer body; firing the multilayer body to obtain a sintered body; and heat treating the sintered body in an atmosphere of an oxygen partial pressure of at least 10xe2x88x924 Pa, more preferably 10xe2x88x921 to 10 Pa.
Preferably, the method of producing an electronic device according to the present invention further comprises, in the firing step, firing the multilayer body in an atmosphere of an oxygen partial pressure of 10xe2x88x9210 to 10xe2x88x923 Pa.
Preferably, the method of producing an electronic device according to the present invention uses nickel or a nickel alloy as the internal electrode paste.
Note that in the dielectric ceramic composition according to the present invention, each of the SiO2, MO, Li2O, B2O3, and (Srp, Ca1xe2x88x92p)SiO3 contained in the third subcomponent should form the composition at least after firing. Compounds which form these oxides after firing are also included.
Effects
The dielectric ceramic composition according to the present invention, by adding a predetermined amount of a specific first subcomponent to a main component containing a dielectric oxide of a specific composition, is superior in the resistance to reduction at the time of firing, has a superior capacity-temperature characteristic after firing, and is improved in the accelerated life of the insulation resistance more than 1000-fold (for example, 175xc2x0 C., DC8V/xcexcm) compared with when not adding the first subcomponent.
The chip capacitor or other electronic device according to the present invention has a dielectric layer comprised of a dielectric ceramic composition according to the present invention, so has a superior temperature coefficient of capacitance (for example, satisfies the SL characteristic of the JIS standard) and is improved in the accelerated life of the insulation resistance and improved in the reliability of the electronic device.
The method of producing an electronic device according to the present invention has a step of heat treating a sintered body obtained by firing in an atmosphere of an oxygen partial pressure of at least 10xe2x88x924 Pa, so can effectively improve the defect rate of the initial insulation resistance while maintaining the reliability sought for a chip capacitor or other electronic device.
The present disclosure relates to subject matter contained in Japanese Patent Application No. 2000-31802 (filed on February 9) and No. 2000-187799 (filed on June 22), the disclosure of which is expressly incorporated herein by reference in its entirety.